This work is aimed at obtaining data concering the lifetime and reactivity parameters of excited molecular oxygen (singlet oxygen) in milieux of relevance in biology and health. A wealth of information is known about singlet oxygen in physico-chemical systems; such data have been used in attempts to interpret oxidation phenomena in biological systems as proceeding via singlet oxygen intermediates. More precise and relevant kinetic data are required for micellar, liposomal and cellular systems in order to be able to confidently infer molecular mechanisms of photo-induced oxidative damage in living systems.